How you enforce your contractual agreement really depend on the conditions, terms, and language of the contract. who has title to the vehicle? our advice is that you sue him/her and get a court order under your arm before you do anything. Do not take the laws into your hands.
If you had an agreement with someone to buy a car as a private seller, it is always a good idea to get something in writing. If they did not pay for the vehicle, you can file a claim in small claims court to get the vehicle back or to get the money owed.
If you have the Title and Registration and these are in YOUR NAME, then by law it's YOUR CAR! That's what titles are for....proof of ownership! If you gave the deadbeat the Title, then you'll have to sue to prove ownership. Good luck!